A number of solvent-free polymer electrolytes are known and there has been considerable interest in the possible utilization of the electrolytes in electrochemical devices such as batteries, fuel cells, supercapacitors, electrochromic devices and sensors. Among the polymers which have been tested for such use are those based upon the linear-chain polyethers, poly(ethyleneoxide) and poly(propyleneoxide), with alkali metal salts. Phosphazene and siloxane comb polymers have also been reported which exhibit better conductivity at room temperature than do the linear-chain polyether electrolytes. One class of polymers of interest are the polyphosphazene sulfonates as reported by S. Ganapathiappan, Kaimin Chen and D. F. Shriver, Macromolecules, 1988, 21, 2299, in Journal of the American Chemical Society, 1989, 111, 4091 and Chemistry of Materials, 1989, 1, 483. Polyether electrolytes are reported in, for example, Polymer Communications, 1987, 28, 302. Polyester conductive polymers are reported in, for example, Macromolecules 1988, 21, 96. Cation conductive siloxane comb polymers are reported in Polymer Communications, 1989, 30, 52 and in Journal of Polymer Science: Part C: Polymer letters, 22, 187-191, 1990. Anionic conductivity is also known in solid polymer electrolytes as is reported, for example, in Macromolecules 1984, 17, 975.
While the various polymer electrolytes set forth in the above publications have shown promise, such promise has generally not been enough to make them practical choices for use in, applications wherein it is desirable to have particularly high ionic conductivity for the polymer electrolyte and wherein it is desirable to use relatively thin films of the polymer electrolyte. Basically, the polymer electrolytes of the prior art do not exhibit sufficient ionic conductivity. Furthermore, the polymer electrolytes of the prior art have generally not exhibited desirable physical properties for incorporation in electrolytic devices. For example, the films may be too sticky, the polymers may be too close to being liquid, the polymers may be too brittle, or the polymers may be too heat sensitive.
A number of plasticizers have been found to be useful for enhancing the ionic conductivity of solid polymer electrolytes. However, the plasticizers of the prior art often suffer from various drawbacks. For example, some of them are too volatile and will escape the polymer electrolyte over a period of time whereby the conductivity will decrease and the properties of the polymer will likewise change, for example the polymer might become more brittle and/or might peel from a substrate on which it has been coated. Plasticizers which would provide the desired plasticizing effect, would not be readily volatilized away from the polymer, which will not deleteriously alter the mechanical properties of the polymer and which would significantly enhance the ionic conductivity of the solid polymer electrolyte would therefore be desirable.
The present invention is directed to overcoming one or more of the problems as set forth above.